That would cause any particular user to leak what user on the site they were. In other words, if I connect with https to lavabit normally, anyone watching my c

That would cause any particular user to leak what user on the site they were. In other words, if I connect with https to lavabit normally, anyone watching my connection knows I went to lavabit, but don't know what account I am associated with. If I have to hit a subdomain specific to my user, then they know.